As is well known, so-called horizontal axis wind turbines are used broadly in wind-powered electric power generating equipment. Typically the horizontal axis wind turbine is structured comprising a rotor to which at least blades are attached radially from a hub, a nacelle wherein a rotor is supported rotatably through a main axis that extends in essentially the horizontal direction, to which the hub is attached, and a tower that supports the nacelle rotatably in the yaw direction, and which is disposed in essentially the vertical direction.
In addition, a yaw driving means capable of drive-controlling the yaw rotation of the nacelle, and a control means such as a yaw brake that brakes the yaw rotation, or a main shaft brake that brakes the rotation of the rotor have conventionally been employed in a horizontal axis wind turbine. Moreover, horizontal axis wind turbines having means for controlling the pitch angle of the blades are also being used.
One kind of horizontal axis wind turbine is a downwind type of horizontal axis wind turbine that is constructed such that when the force of the wind on the blades rotates the rotor, the rotor is located further on the downwind side than the tower. On the other hand, an upwind type of horizontal axis wind turbine is constructed such that when the force of the wind on the blades rotates the rotor, the rotor is located further on the upwind side than the tower. In Japanese Patent Application Publication No. 2005-61963 and Japanese Patent Application Publication No. 2006-329107 a downwind type of horizontal axis wind turbine is disclosed in which a anemometer is provided in the nacelle.
The amount of electric power that is generated by wind-power generation and the cost of that electric power strongly relies on the scale of the wind turbine and the wind speed, so there is a trend to continuously increase the size of the wind turbines and to collectively install wind turbines in large open areas where there are high wind speeds as wind farms. Japan is a mountainous, narrow country comprising densely populated areas, so land that is suitable for wind-power generation is spread over complicated geography such as hills.
Furthermore, the durability and the performance of a wind turbine are greatly affected by the turbulence intensity. When the intensity of the turbulence is greater than what the wind turbine was designed for, there is a tendency for fatigue loading to increase, which causes an increase in fatigue damage and a drop in durability.
In addition, from the aspect of performance, there is a tendency that the output near the rated output will be decreased. Wind farms mentioned above, and environments having complicated geography tend to increase the intensity of the turbulence, so when evaluating the durability and performance of a wind turbine, in addition to knowing the wind speed at each wind turbine location, it is also essential that the intensity of the turbulence at each location be known.
Normally, a nacelle anemometer that is located on the nacelle is only used when starting or stopping control of the wind turbine, however, the anemometer is also often used in evaluation of the performance.
However, it is necessary that the wind speed characteristics that must be obtained for the design of the wind turbine be aimed at the airflow to the rotor.
Therefore, conventionally, in order to obtain the value of the wind speed of the airflow to the rotor of the wind turbine, a mast (hereafter, referred to as a reference mast) is installed at a location with a high certainty of having the same wind conditions as the wind turbine, and an anemometer that is placed on the mast at a height that is nearly the same height as hub of the wind turbine is used.
The average wind speed in the case of a nearly horizontal wind, can be corrected to an equivalent to the average wind speed of air flowing to the rotor by taking a correlation between the wind speed that is measured by the anemometer on the nacelle during test operation, and the wind speed that is measured by the anemometer on the reference mast.
The turbulence intensity as well, can feasibly be corrected to an equivalent to the turbulence intensity of the airflow to the rotor by taking the correlation between the wind speed that is measured by the anemometer on the nacelle during test operation, and the wind speed that is measured by the anemometer on the reference mast.
However, a normal upwind type of horizontal axis wind turbine has an anemometer that is located on the downwind side of the rotor, so it is not possible to obtain effective data for the turbulence intensity. In other words, it is not possible to evaluate the output and durability while taking into consideration the intensity of the turbulence.
Therefore, the inventors of the present invention experimented with measuring the turbulence intensity of airflow to the rotor by using an anemometer on a nacelle that is located on the upwind side of the rotor of a downwind type of horizontal axis wind turbine.
The nacelle is moved by the wind that the wind turbine receives. While the nacelle is moving, the values obtained from the anemometer on the nacelle deviates from the value that is measured when the nacelle is in a non-moving state, or in other words, deviates from the absolute value by the amount of the speed of movement of the nacelle. However, movement of the nacelle is unavoidable during measurement. Therefore, even when obtaining the value of the absolute turbulence intensity, it is necessary to take into consideration the effects of movement of the nacelle with respect to the measured value from the anemometer on the nacelle.
Taking the problems of the related art into consideration, the present invention aims to provide a method for measuring the turbulence intensity of a horizontal axis wind turbine that is capable of more accurately obtaining the absolute value of the turbulence intensity where the effect of the rotor is small and the effect of the movement of the nacelle is taken into account.